The invention relates to a radiation-emitting diode with improved radiation output, and to a method for manufacture thereof. In particular, the invention relates to an infrared diode with a surface toughened to improve the radiation emission.
Infrared-emitting diodes of silicon-doped gallium arsenide are used on a large scale today as transmitters for a wide variety of remote control applications. To ensure high functional reliability and a low power requirement, as great as possible an external quantum efficiency is required from the diodes.
The external quantum efficiency of a light-emitting diode is determined both by the internal quantum efficiency and by the losses occurring when radiation leaves the interior of the diode. One of the main causes for such losses is the radiation proportion resulting from the high optical refraction coefficient of the semiconductor material--about 3.6 for gallium arsenide--that cannot be emitted at the semiconductor surface on account of total reflection. In the case of gallium arsenide, a critical angle for the total reflection of 16.2.degree. results at the transition to air. By the direct path, only the proportion of radiation that hits the boundary surface below a lower angle to the surface normal is emitted. This radiation proportion is however still subject to a partial reflection caused by the abrupt change in the refraction coefficient. For radiation hitting the boundary surface vertically, the transmission coefficient is about 68%, so that if the absorption of the radiation on the way to the boundary surface is ignored, only about 2.7% of the generated radiation can leave the semiconductor crystal on a direct path in the case of a flat structure.
The radiation emission from the interior of the diode can be improved by various measures, such as application of a .lambda./4 thick coating, covering with a material adapted to the refraction coefficient, roughening of the diode surface, or a combination of the above measures.
From EP 404 565 (U.S. Pat No. 5,040,044), a radiation-emitting diode made from a III-V compound semiconductor material is known in which the entire surface of the semiconductor chip is roughened in order to improve the external quantum efficiency. Roughening prevents total reflection of the generated radiation at the boundary layer between the diode chip and the surrounded material, shortens the light travel distance inside the semiconductor material, and hence reduces the probability of reabsorption. However, a drawback of a semiconductor chip surface roughened in this way is that the etched contact surface makes the chip very difficult to bond. In addition, etching of the surface in the vicinity of the radiation-emitting pn-junction leads to reduced service lives of the diodes.